1. Field of Invention
The present invention relates to a process for separating molten metals, and more particularly to a process for separating molten metals that are treated with metal hydrides.
2. Background of the Invention
As is well known in the art, magnesium, aluminum and zinc, as well as their alloys, and numerous other nonprecious metals become covered with a more or less protective oxide skin (passivation) even under environmental influences. This skin formation may cause undesirable gray discoloration of the metals. This is seen in the case of aluminum, wherein it is known that impurities such as iron, silicon and other foreign metals, as well as reaction products thereof, reduce the transparency of the oxide film which is formed and impart a matt gray color to the surface (Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Ed., Vol. 2, p. 247).
On the other hand, the passivity of nonprecious metal oxide layers opens up numerous applications to such metals. Treating the metals with reducing agents, e.g., with nascent hydrogen and hydrogen gas at elevated temperatures, leads to a loss of passivity (Rxc3x6mpps Chemie Lexikon, 9th Edition (1991), p. 3230).
The present invention is directed to a process for separating molten metals selected from Groups II-IV of the Periodic Table of Elements or their alloys by treating the molten metal or alloy thereof with a metal hydride. The process of the present invention produces metals which have a highly reflective surface, yet are highly resistant to tarnishing. That is, the metals produced in the present invention are passivated and are not adversely affected by customary environmental influences.
A first embodiment of the present invention comprises a process for separating molten metals of principal groups II-IV of the Periodic Table of Elements as well as their subgroups and alloys, wherein the metals or alloys are reacted with a metal hydride in a molten bath.
When metals or alloys are melted in the presence of metal hydride, a separation of metal or alloy constituents is observed. This is revealed, for example, by the formation of a grainy layer covering the molten material or the solidified regulus. Following this treatment, the metals or metal alloys exhibit a changed chemical composition compared to the untreated starting material.
In a preferred embodiment of the present invention, the metals are selected from those which, under standard environmental conditions, are subject to external passivation. In particular, the metals employed in the present invention are metals which form an oxide skin under standard environmental conditions. Therefore, it is preferable in the context of the present invention for these metals to be selected from nonferrous metals and nonprecious metals, and more preferably selected from magnesium, calcium, aluminum, silicon, titanium or zinc, and their alloys. The term xe2x80x9calloyxe2x80x9d is used in the context of the present invention to denote a composition that contains at least 30% by weight of said metal.
Another embodiment of the present invention comprises reacting metals or metal alloys with metal hydrides of one of the metals which are to be separated. For example, to separate magnesium, it is particularly advantageous to use magnesium hydride. Similarly, of course, it is also possible to use mixed metal hydrides in the case of alloys. In the same way, however, it is also possible to deliberately introduce new material components into the metal alloy by specifically selecting the metal hydride or hydrides.
A still further embodiment of the present invention consists of setting a molar ratio of metal, including alloys, to metal hydride in the range from 1:0.0001 to 1:100, preferably in the range from 1:0.001 to 1:0.01, and even more preferably in the range from 1:0.005 to 1:0.03. By carrying out the reaction of the present invention in the ranges mentioned above, repeated treatment is typically unnecessary. Thus, the present invention significantly reduces the cost of separating molten metals. This provides an economic advantage over prior art processes.
Using the present invention it is therefore possible to remove impurities, in particular, volatile impurities which form metal hydride, from a metal.
The metals or alloys are treated with the metal hydrides in the molten state. It is advantageous to use the hydrides whose metals also act as alloying components in the metal matrix.
The present invention unexpectedly provides a metal which has a reflective surface, yet is passivated such that unwanted reactions caused by customary environments, i.e., tarnishing, does not occur.
For example, for lamp reflectors and shiny decorative effects in the automotive and mechanical engineering sectors, it is desired to use high-purity alloys which guarantee maximum reflection and shine (Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Ed., Vol. 2, p. 247). The purity of the metal, and its surfaces, is therefore directly linked to the observed shine and reflectivity, so that it is possible to deduce directly from this relationship that the metals obtained using the process according to the present invention are of very high purity. In view of the fact that commercially available metals are often used with a purity of almost 100%, and these metals are passivated in relation to environmental atmospheres such as air using separate processes, the process according to the present invention is an efficient refining process, for example, for the removal of impurities, and especially, volatile impurities which form metal hydrides.
For example, if a small amount of autocatalytically produced magnesium hydride (TEGO-Magnan(copyright)) is applied to a pulverulent Alxe2x80x94Mg alloy, and this mixture is heated in a muffle to approx. 1000xc2x0 C., the solidified metal, which is obtained after the reaction mixture has cooled, has shiny, bright silver-colored external and internal surfaces which do not become tarnished even after they have been stored for months in ordinary air. It was also possible to make a similar observation when zinc (electrolytic zinc) was reacted with magnesium hydride. In addition to a shiny silver-colored surface which was stable under environmental atmospheres, the impurity levels of lead, tin, iron and copper had fallen considerably.